1. Field of the Invention
This invention relates to a control system for a gas turbine engine and, more particularly, to an improved integrated control system including a primary control, a parallel operating back-up control, the latter providing for continued operation of the engine in the event of a malfunction or failure of the primary control.
2. Description of the Prior Art
The need for the electronic engine control relates fundamentally to the increased complexity, i.e., number of controlled engine variables, inherent cycle variability, inlet-engine-exhaust control integration, of the next generation of sophisticated variable cycle engines. This level of engine complexity and the expanding need for more complete aircraft/engine control integration forces a commensurate increase in the complexity and required capability of the control system. This increased control system capability is needed to provide stable, responsive, safe, and precise engine control. Digital electronics represents the only viable means of meeting the significantly more sophisticated requirements of the variable cycle engine of the future. With its inherent ability to "time-share" in the computational section of the control, increasing requirement complexity yields a very much smaller increase in hardware when compared to present control system mechanizations. Digital electronics also lends itself to "hardening" to the aircraft engine environment through hybrid construction, packaging, and environmental conditioning techniques.
Full authority electronic control systems for gas turbine engines provide a number of significant advantages over conventional hydromechanical control systems. However, since full authority electronic control systems are only advancing to what might be termed a first generation level, confidence in the reliability of such electronic controls has not attained the level generally associated with the hydromechanical systems which have been in use for many years in highly refined configurations. Consequently, it is advisable to complement the primary electronic control with a secondary or back-up control which will assure continued operation of the engine in the event the primary electrical control exhibits a malfunction or a failure.
Prior art control systems, for example the "Integrated Control System for a Gas Turbine Engine" as disclosed in U.S. Pat. No. 4,137,707, have typically comprised a primary electronic control in combination with a hydromechanical back-up control. Although such systems function well, they are relatively complex in terms of hardware and add unnecessary weight to the control system. In addition, such systems are relatively expensive to produce.
Another problem associated with some prior art control systems is that when switching from the primary control to the back-up control (or vice versa) the change may result in a step-type change causing an immediate alteration in the operation of the engine. It is also possible that uncontrolled transients could be introduced into the engine as control is transferred. Both step-type changes and transients in the engine are undesirable since they may require immediate operator attention at a time when the operator can least afford it.
It is, therefore, an object of the present invention to provide an integrated control system which is capable of reliably controlling a relatively complex gas turbine engine.
It is another object of the present invention to provide such a control system which is relatively lightweight and inexpensive to produce.
It is a further object of the present invention to provide such a system which is fully compatible with the present trend toward fully electronic controls.